In a rear seat side airbag device disclosed in the below-identified Patent Document 1, an airbag module is disposed inside a deck side trim provided at a vehicle width direction outer side of a seatback of a rear seat. A tear portion is specified in the deck side trim. When an inflation pressure of the side airbag reaches a predetermined value, the tear portion ruptures and the side airbag is deployed to a vehicle forward side thereof    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2010-120407